1. Technical Field of the Invention
The present invention relates to a rotary electric apparatus mounted on vehicles such as cars and autotrucks.
2. Description of Related Art
It has been strongly desired that rotary electric apparatuses for vehicles be light in weight and high-performance so that power to be consumed can be minimized. However, it is required for rotary electric apparatuses under operation to change their rotation speeds, which has made it difficult to elevate their performances. For example, in cases where the electric machine is operated as a generator, the number of rotations of the generator is required to continuously changed from approx. 1500 rpm (corresponding to an vehicle""s idling state) to approx. 15000 rpm (corresponding to a rotation number in a vehicle""s high-speed run). In other words, there is a difference as much as one digit between the upper and lower limits of a range of the number of rotations, so that the rotary electric apparatus under operation is wide in a rotational operation range.
Such a feature that the on-vehicle rotary electric apparatus involves a wide operation range results in contradictory inconveniences as follows. If it is desired that output current be increased at lower idling rotation speeds, one choice is to increase the number of windings of a stator coil. However, when such a measure is adopted, the inductance value of the stator coil is obliged to be larger, resulting in less output current at higher rotation speeds due to the increase in the inner impedance. By contrast, when the number of windings of the stator coil, the output current at higher rotation speeds will be raised, but the output current at lower rotation speeds is forced to be reduced on account of lower inductive voltage.
Meanwhile, when the apparatus is adapted to operate as a motor, the identical situation to the above generator will be posed. To be specific, if the number of windings of a stator coil, a larger torque can be gained at higher rotation speeds due to the fact that a back electromotive force is small. However, at lower rotation speeds, a magnetomotive force is forced to become smaller, so that the torque is made smaller correspondingly. On the other hand, when the number of turns of the stator coil are set to a larger number, the opposite relationship to the above is given.
As described above, the on-vehicle rotary electric apparatus, which can be used as a generator or a motor, requires a wide range of rotation speeds during its operation. Hence there arises the problem that it is very difficult to attain higher output performances at both of lower and higher rotation speeds.
The present invention has been performed in consideration of the above-described problem, and an object of the present invention is to provide an on-vehicle rotary electric apparatus capable of attaining highly improved output characteristics in both of lower and higher rotation speeds, that is, in the whole rotational speed range.
The present inventor found that, in both the generator and motor, the foregoing problem was attributable to the fact that voltage induced across the stator winding and/or reactance of the stator winding are uniquely proportional to the rotation speed of the rotor. An inventor""s study showed that a key to the resolution of the problem was derived from the control of the induced voltage and reactance irrelevantly to the rotation speed of the rotor.
An on-vehicle rotary electric apparatus according to the present invention comprises a stator having multi-phase windings and a rotor having one or more field windings for producing a rotating magnetic field in response to supply of exciting current to the field windings. The rotor is driven to rotate by either an on-vehicle motor or a running drive shaft of a vehicle. The apparatus further comprises a driving member for driving the field windings by supplying the exciting current to the field windings and changing the exciting current so that the rotating magnetic field produced by the rotor rotates at a speed different from a rotation speed of the rotor.
Conventionally, an alternating frequency of N/S magnetic poles has been dependent on the rotation speed of a rotor. However, changing the speed of a rotating magnetic field produced at the rotor allows the alternating frequency of N/S magnetic poles to be changed independently of an actual rotation speed of the rotor. Hence the induced voltage, which is dependent on the frequency of an alternating magnetic field, can be changed separately from an actual number of rotations of the rotor. Also, the reactance of the stator winding depends on the frequency of an alternating magnetic field across the stator winding. Hence, as to this reactance, it can be changed independently of an actual number of rotations of the rotor.
This configuration can be applied to for example an AC generator. When the generator rotates at lower speeds, the rotating magnetic field is accelerated in a rotating direction of the rotor to increase the field. As a result, higher voltage is generated even at lower rotation speeds, thus providing a larger output. By contrast, in cases where the generator is forced to rotate at higher speeds, the rotating magnetic field is prohibited from rotating relatively to the rotor or made to rotate even in the reverse direction to the rotor. Hence a relative speed between the stator armature and the rotating magnetic field can be lowered, thus lessening an armature counteraction (that is, a reactance), thus raising the output.
Preferably, the driving member comprises short-circuit windings wound around part of magnetic poles of the rotor and a current-supply unit for supplying, as the exciting current, a single-phase alternating current to the field windings. Further, the short-circuit windings produce a magnetic field delayed in phase by substantially 90 degrees compared to the magnetic field produced by the rotor. That is, supplying the single-phase alternating current makes it possible to excite the rotor.
Conventionally, in order to obtain the foregoing rotating magnetic field, it was necessary to apply to the rotor winding three-phase alternating current or two-phase alternating current with a phase difference. This current-supplying configuration necessitates the arrangement of many semiconductor switches. In contrast with the conventional, the present invention employs the single-phase alternating current to excite the rotor, thus requiring a less number of semiconductor switches. Concurrently, in order to make it possible that the above single-phase excitation produce a rotating magnetic filed, the short-circuit coils are wound around part of the magnetic poles of the rotor. The short-circuit coils produce a second magnetic field delayed in phase by 90 degrees from the single-phase alternating magnetic field, thereby producing a moving magnetic field, that is, a rotating magnetic field. Compared to the conventional way whereby three-phase alternating current or two-phase alternating current with a phase difference was used to produce the rotating magnetic field, the control of on/off operations of the semiconductor switching elements can be simplified. It is also possible to reduce electrical parts such as semiconductor switches.
Alternatively, the current-supply unit may include an H-type of bridge circuit consisting of four switching elements, two of the four switching elements being connected in a crossed connection to each other with the field windings therebetween, and a field controller for controlling on/off operations of the four switching elements so that the single-phase alternating current passing the field windings change alternately in a passing direction thereof. This circuitry allows a single-phase alternating current to be fed to the field windings, whereby providing the single-phase excitation to the rotor, with the rotating magnetic field still produced.
It is also preferred that the field windings are composed of two partial field windings each of through which the single-phase alternating current is supplied in a one-way direction, in which the one-way direction assigned to each of the partial field windings is different one from the other. In this case, the current-supply unit includes two switching elements each connected to the two partial field windings and a field controller for controlling on/off operations of the two switching elements so that the single-phase alternating current alternately passes through each partial winding in each of the two one-way directions. In particular, this configuration makes it possible to reduce the number of switching elements than the circuitry using the foregoing H-type of bridge circuit.
Still alternatively, the field windings may be connected in parallel to a capacitor so as to form a parallel circuit having a resonant frequency. In this case, the current-supply unit includes a single switching element each connected to the parallel circuit and a field controller for controlling on/off operations of the switching element so that the single-phase alternating current passes through the field windings in synchronism with the resonant frequency. The resonance of the parallel circuit allows the current-supply direction through the field winding to change periodically and the number of switching elements can be minimized.